Teach config.guess that MSYS exists.

We might not want to upgrade config.guess to the current
version due to the license change from GPL2 to GPL3.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242178 91177308-0d34-0410-b5e6-96231b3b80d8

autoconf/config.guess

@@ -810,6 +810,9 @@ EOF
     *:MINGW*:*)
 	echo ${UNAME_MACHINE}-pc-mingw32
 	exit ;;
+    *:MSYS*:*)
+	echo ${UNAME_MACHINE}-pc-msys
+	exit ;;
     i*:windows32*:*)
 	# uname -m includes "-pc" on this system.
 	echo ${UNAME_MACHINE}-mingw32